These cards have a 50MHz oscillator that gives a 200MHz machine through a 4x multiplier, so the bus is running at 50MHz, and that leaves a small area to work with since bus maxes are around 60MHz these days. I have two reports of a 200MHz card being clocked to 220MHz with a 55MHz oscillator and one report of 229.2MHz with a 57.3MHz oscillator. Here too, the L2 cache may be an important factor to keep in mind as some of the slower cache cards cannot be accelerated very far. Mod 1 and Mod 2 are the best ways to clock chip these machines since the initial oscillators are the metal can type. I've heard from one person who treid a 60MHz oscillator, and got a 240MHz machine once he added some extra cooling for the processor. As a reference, this 60/240MHz configuration gave a MacBench 4.0 CPU score of 573. Apparently there is also a resistor modification you can follow to change the 4x multiplier to 3.5x (and probably others), and with a 60MHz oscillator that gave a 60MHz bus and a 210MHz 604e card.
Also check out the Bus:Processor Modification on Takashi Imai's web site.